330 ANIMAL AGGREGATIONS 



enemies, or by changed environmental conditions or overcrowding, 

 and the species persists in the solitary phase. When conditions are 

 again favorable, as at the end of a severe drought, these scattered 

 grasshoppers multiply rapidly. As numbers permit, they gather 

 into loose swarms, which deposit eggs in compact lots. From these 

 compact deposits the swarming type of nymphs arises. 



Another link in the chain of evidence connecting the two phases 

 is furnished by Johnson (1926), working with the desert locust 

 Schistocerca gregaria and S. flaviventris. Uvarov (1923), as a result 

 of his studies on the phases of Locusta migratoria, suggested, from his 

 inspection of museum specimens of these two supposedly good spe- 

 cies of desert locust, that Schistocera gregaria was the swarm-phase 

 and S. flaviventris the solitary phase of one and the same species. 

 Field observations recorded by Johnson suggested that this was 

 indeed the case, and these observations were followed by breeding 

 experiments which gave the same results as have similar tests with 

 other species, since the experimenter was able to control the appear- 

 ance of either phase by regulating the density of the crowding. A 

 part of this test was made in nature and on a large scale. Bands of 

 hoppers thinned out by poison turned into the solitary phase so far 

 as coloration and behavior were concerned. There is also prelimi- 

 nary evidence (Dampf, 1925, 1926) that the South American locust 

 5. paranensis can be turned into the solitary phase usually known as 

 S. americana by controlling the density of the population. 



The phase theory of Uvarov remains to be tested even in a pre- 

 liminary manner upon two other species, which, with the four dis- 

 cussed, make up the larger swarming locusts of the world. These 

 are the African red locust Nomadacris septemfasciata, of which two 

 types are known, differing in pronotal characters; and Patangia 

 succincta of India, which apparently rarely swarms. Of the smaller 

 swarming locusts, there is a suggestion that the Moroccan locust, 

 Dociostaurus maroccanus, has a solitary phase which has been de- 

 scribed as a pigmy race. Finally, there is the case of the Rocky 

 Mountain locust, Melanoplus spretus, now apparently extinct in its 

 typical form. In 1878, the United States Entomological Commis- 

 sion, composed of Riley, Packard, and Thomas, in their first annual 



